Relatively small (e.g., 3 to 5 IRE) signals encoding digital information can be admixed together with composite video signals without being objectionably evident in television pictures generated from those composite video signals if suitable restrictions on the digital signal format are observed. This is pointed out by A. L. R. Limberg, C. B. Patel and T. Liu in their U.S. patent application Ser. No. 08/108,311 filed 20 Aug. 1993, entitled APPARATUS FOR PROCESSING MODIFIED NTSC TELEVISION SIGNALS, WITH DIGITAL SIGNALS BURIED THEREWITHIN, and incorporated by reference herein. Limberg et alii describe phase-shift-keyed (PSK) modulation of a subcarrier having a frequency that is an odd multiple of one-half scan line frequency. The phase shift keying is done responsive to serial-bit digital data supplied at a symbol rate that is a multiple of one scan line frequency. Limberg et alii prefer repeating frames of the modulated subcarrier in antiphase in successive pairs of consecutive frames of the NTSC television signal. Because of frame-averaging effects resulting from the limitations on the speeds of the response of the human visual system and the decay of electroluminescence of kinescope phosphors, such repetition of data in pairs of frames makes PSK subcarrier accompanying the composite video signal detected from the NTSC television signal less visible in images that are generated from the composite video signal for viewing on a screen. Such repetition of data in pairs of frames also provides a basis for using frame-comb filtering in a digital signal receiver to separate PSK subcarrier from the luminance portion of the composite video signal that describes static portions of successive television images. Limberg et alii prefer also repeating the modulation of the digital data in antiphase in contiguous pairs of adjoining scan lines of the NTSC television signal, providing a basis for using line-comb filtering in the digital signal receiver to separate PSK subcarrier from the chrominance portion of the composite video signal.
Limberg et alii describe a suppressed, vestigial-sideband, amplitude-modulated (VSB AM) carrier of the same frequency as the VSB AM picture carrier, but in quadrature phasing therewith, being used to transmit a suppressed subcarrier modulated with digital data. The modulation scheme Limberg et alii prefer is binary phase-shift-keying of a PSK carrier, a single-sideband (SSB) of which is selected for translation in frequency to form an upper sideband of a suppressed subcarrier that is at a frequency which is a small odd multiple of one-half scan line frequency. In each of the digital signal receivers described by Limberg et alii synchronous detection of the quadrature-phase VSB AM carrier recovers the digital subcarrier, without substantial accompanying composite video signal energy in the baseband extending up to 0.75 MHz frequency. Above 0.75 MHz the VSB AM video carrier begins the transition from being a double-sideband amplitude-modulated (DSB AM) carrier to being a single-sideband amplitude-modulated (SSB AM) carrier. The composite video signal is detected with gradually increasing efficiency up to the 1.25 MHz frequency at which roll-off of the vestigial sideband is complete. Over the same 0.75 to 1.25 MHz frequency range the efficiency with which the digital subcarrier is detected gradually decreases to half its value below 0.75 MHz. A synchronous video detector detecting the quadrature-phase VSB AM video carrier will, providing the intermediate-frequency (IF) amplifier passes the vestigial sideband, generate a response to the PSK subcarrier and remnants of NTSC composite video signal that does not include direct components or synchronizing pulses. This reduces the dynamic range of the synchronous video detector response to the quadrature-phase VSB AM video carrier, easing the problem of digitizing the response without losing low-level PSK subcarriers because of quantization effects.
Limberg et alii describe a digital signal receiver in which the synchronous video detector for quadrature-phase VSB AM video carrier is followed by a cascade connection of a lowpass line-comb filter and a highpass frame-comb filter. The lowpass line-comb filter is for separating the frequency spectrum of a PSK subcarrier having a frequency that is an odd multiple of half-scan-line frequency from chrominance signal portions of the frequency spectrum of an NTSC signal, particularly of an NTSC signal that has been appropriately pre-filtered. The highpass frame-comb filter is for separating the frequency spectrum of a PSK subcarrier having a frequency that is an odd multiple of half-scan-line frequency from motion-free luminance signal portions of the frequency spectrum of an NTSC signal. Limberg et alii teach that the remnant spectrum of the NTSC signal in the response of the cascaded highpass comb filters can be viewed as the frequency spectrum of a jamming signal accompanying the PSK signal. Accordingly, the remnant spectrum of the NTSC signal in the response of the cascaded highpass comb filters can be discriminated against by synchronous symbol detection.
Two different types of partial-response filtering are of interest in regard to the inventions disclosed in this specification and its drawing. Digital delay lines as used in these filters are commonly constructed using random-access memories (RAMs) arranged for operation in a read-then-write-over mode while being addressed by an address counter counting the number of samples per line in the case of a "1-H" digital delay line or counting the number of samples per frame in the case of a "1-F" digital delay line.
A first type of partial-response filter of particular interest is referred to as a "line-comb" partial-response filter in this specification. This filter is composed of one or more sections, each of which sections includes a respective two-input exclusive-OR (XOR) gate having a first input for receiving serial-bit data for partial-response filtering and an output from which section response is taken. Each section further includes a "1-H" digital delay line for applying the section response, as delayed by "1-H" which is the duration of one horizontal scan line in an NTSC television signal, to the second input of the XOR gate in that section. A "line-comb" partial-response filter referred to more specifically as a "pre-line-comb" partial-response filter precedes a line-comb filter, and a "line-comb" partial-response filter referred to more specifically as a "post-line-comb" partial-response filter follows a line-comb filter and is of particular interest in regard to the inventions herein described.
A second type of partial-response filter of particular interest is referred to as a "frame-comb" partial-response filter in this specification. This filter is composed of one or more sections, each of which sections includes a respective two-input exclusive-OR (XOR) gate having a first input for receiving serial-bit data for partial-response filtering and an output from which section response is taken. Each section further includes a "1-F" digital delay line for applying the section response, as delayed by "1-F" which is the duration of one frame of NTSC television signal, to the second input of the XOR gate in that section. A "frame-comb" partial-response filter referred to more specifically as a "pre-frame-comb" partial-response filter precedes a frame-comb filter, and a "frame-comb" partial-response filter referred to more specifically as a "post-frame-comb" partial-response filter follows a frame-comb filter and is of particular interest in regard to the inventions herein described.
J. Yang describes binary phase-shift-keyed (BPSK) modulation of a suppressed carrier that is the same frequency as a video carrier and is in quadrature phasing therewith in his U.S. patent application Ser. No. 08/141,070, filed 26 Oct. 1993, entitled APPARATUS FOR PROCESSING NTSC TV SIGNALS HAVING DIGITAL SIGNALS ON QUADRATURE-PHASE VIDEO CARRIER and incorporated herein by reference. Yang also advocates repeating frames of the BPSK in antiphase in successive pairs of consecutive frames of the NTSC television signal, just as Limberg et alii do. Yang advocates the BPSK signals being constrained to about 2 MHz bandwidth, so as to avoid crosstalk into chroma in TV receivers that separate chroma from luma without recourse to comb filtering. Yang indicates a preference for passing the data to be transmitted through a pre-line-comb partial-response filter prior to its digital-to-analog conversion to an analog modulating signal for a balanced amplitude modulator. This is done to preserve the information contained therein when line-comb filtering is done in the digital signal receiver to separate PSK subcarrier from the luminance portion of the composite video signal. Line-comb filtering in the digital signal receiver converts the partial-response filtered binary digital signal to ternary digital signal, if the line-comb filtering is of the two-tap type, linearly combining signals differentially delayed by only the duration of one horizontal scan line of video signal. Line-comb filtering in the digital signal receiver converts the partial-response filtered binary digital signal to five-level digital signal, if the line-comb filtering is of the three-tap type, linearly combining signals differentially delayed by the duration of one horizontal scan line of video signal and by the duration of two horizontal scan lines of video signal. Therefore, multi-level symbol decision circuitry is required to recover bit-serial digital data transmitted by the BPSK from the comb filtering response.
A U.S. patent application Ser. No. 08/179,618 filed 5 Jan. 1994 J. Yang and A. L. R. Limberg, entitled "PRE-FRAME-COMB" AS WELL AS "PRE-LINE-COMB" PARTIAL-RESPONSE FILTERING OF BPSK BURIED IN A TV SIGNAL, describes a pre-frame-comb partial-response filter as well as pre-line-comb partial-response filtering being used at the digital signal transmitter. Line-comb filtering in the digital signal receiver converts the partial-response filtered binary digital signal to five-level digital signal, if the line-comb filtering is of the two-tap type, linearly combining signals differentially delayed by only the duration of one horizontal scan line of video signal. Line-comb filtering in the digital signal receiver converts the partial-response filtered binary digital signal to nine-level digital signal, if the line-comb filtering is of the three-tap type, linearly combining signals differentially delayed by the duration of one horizontal scan line of video signal and by the duration of two horizontal scan lines of video signal.
Receivers for the Yang system are also described by T. V. Bolger in his U.S. patent application Ser. No. 08/141,071, filed 26 Oct. 1993, entitled RECEIVER WITH OVERSAMPLING ANALOG-TO-DIGITAL CONVERSION FOR DIGITAL SIGNALS WITHIN TV SIGNALS, and incorporated herein by reference. These receivers digitize the response of a quadrature-phase video detector using an oversampling analog-to-digital converter. The digitized quadrature-phase video detector response is subjected to digital frame-comb and line-comb filtering to suppress remnant composite video signals; the comb filtering response is supplied to multi-level symbol decision circuitry to recover bit-serial digital data transmitted by the BPSK; and the bit-serial digital data is supplied to a decoder that corrects the digital information in the data using forward-error-correcting codes contained therein.
Receivers for the Yang system are also described by J. Yang, T. V. Bolger and A. L. R. Limberg in their U.S. patent application Ser. No. 08/179,586 filed 5 Jan. 1994 entitled RECEIVER WITH SIGMA-DELTA ANALOG-TO-DIGITAL CONVERSION FOR DIGITAL SIGNALS BURIED IN TV SIGNALS, and incorporated herein by reference. These receivers digitize the response of a quadrature-phase video detector using an oversampling analog-to-digital converter of sigma-delta type. Preferably, the bit resolution of a basic multiple-bit-resolution flash converter is improved by using a sigma-delta procedure in which only a single bit of the basic multiple-bit-resolution ADC output signal is converted back to analog signal for feedback purposes during each oversampling step, as described by T. C. Leslie and B. Singh in their paper "An Improved Sigma-Delta Modulator Architecture", 1990 IEEE SYMPOSIUM ON CIRCUITS & SYSTEMS, 90 CH 2868-8900000-0372, pp. 372-375, incorporated herein by reference. The digitized quadrature-phase video detector response is subjected to digital frame-comb and line-comb filtering to suppress remnant composite video signals; the comb filtering response is supplied to multi-level symbol decision circuitry to recover bit-serial digital data transmitted by the BPSK; and the bit-serial digital data is supplied to a decoder that corrects the digital information in the data using forward-error-correcting codes contained therein.
The inventions described by A. L. R. Limberg et alii, by J. Yang, by T. V. Bolger, by J. Yang and A. L. R. Limberg, and by T. V. Bolger et alii in their respective patent applications, like the inventions described herein, are assigned to Samsung Electronics Co., Ltd., pursuant to pre-existing employee agreements so to assign inventions made within the scope of employment. In these patent applications the bit-serial data used for generating the binary phase-shift-keying signal have been processed at the transmitter so that the data will survive comb filtering procedures, which are carried out in the digital signal receiver for suppressing the composite video signals accompanying the data and tending to act as a jamming signal. With regard to operation of the combined NTSC television and BPSK transmitter, partial-response filtering of the bit-serial data subsequently used for generating the binary phase, shift-keying signal is advocated by each of these U.S. patent applications, except for Ser. No. 08/108,311.